Various information recording techniques have been developed following the increase in volume of information processing in recent years. Particularly, the recording density of HDDs using the magnetic recording technique has been increasing more and more. In order to achieve the high recording density in a magnetic disk for use in a HDD or the like, it is necessary to reduce the size of magnetic crystal grains forming a main recording layer serving to record information signals, and further, to reduce the thickness of the layer. However, in the case of conventionally commercialized magnetic disks of the in-plane magnetic recording type (also called the longitudinal magnetic recording type or the horizontal magnetic recording type), as a result of the reduction in size of magnetic crystal grains, there has arisen a so-called thermal fluctuation phenomenon where the thermal stability of recorded signals is degraded due to superparamagnetism so that the recorded signals are lost, which has thus become an impeding factor for the increase in recording density of the magnetic disks.
Particularly in recent years, the information recording capacity exceeding 120 GB has been required per, for example, 2.5-inch HDD magnetic disk. In order to achieve such an information recording capacity, the information recording density exceeding 200 Gbit/inch2 is required. In order to achieve such an information recording density, there have been proposed magnetic disks of the perpendicular magnetic recording type instead of the above-mentioned in-plane magnetic recording type.
In the case of the perpendicular magnetic recording type, as different from the case of the in-plane magnetic recording type, the easy magnetization axis of a main recording layer is adjusted to be oriented in a direction perpendicular to a surface of a substrate, thereby preventing adjacent magnetizations from facing each other. As compared with the in-plane magnetic recording type, the perpendicular magnetic recording type can suppress the thermal fluctuation phenomenon and thus is suitable for increasing the recording density.
In order to obtain high thermal stability and excellent recording characteristics in such magnetic disks of the perpendicular magnetic recording type, there have been proposed composite media, such as a CGC perpendicular medium (e.g. Non-Patent Document 1), a stacked medium (e.g. Non-Patent Document 2), and a Cap medium (e.g. Non-Patent Document 3), in which a recording layer is formed by two or more layers.    Non-Patent Document 1: Y. Sonobe et al., IEEE Trans. Magn, vol. 37, 1667-1670 (2001)    Non-Patent Document 2: B. N. Piramanayagam, et. al., IEEE Trans. Magn, vol. 41, 3190-3192 (2005)    Non-Patent Document 3: B. A. Acharya, et. al., IEEE Trans. Magn, vol. 41, 3145-3147 (2005)